Guanidinoacetate methyltransferase deficiency (GAMT-deficiency) is an inborn error of creatine metabolism characterized by muscular hypotonia, involuntary extrapiramidal movements and epilepsy. The disease is biochemically characterized by accumulation of guanidinoacetate and deficiency of creatine and phosphocreatine in tissues of affected patients. However, the mechanisms underlying the neurological dysfunction of GAMTdeficiency patients are not well understood. Na+,K+-ATPase, creatine kinase (CK) and acetylcholinesterase (AChE) play a fundamental role in central nervous system (CNS). Alterations in these enzymes with reduction of energy metabolism and induction of oxidative stress have been associated to neurological diseases such as Alzheimer’s, Parkinson’s, Huntington’s disease and brain ischemia. Neurotoxic properties of GAA have been mainly related to excitotoxicity and oxidative stress. In this study we investigated possible pathophysiologic mechanisms of GAMT-deficiency. We studied the in vitro effect of GAA and the effect of GAA intrastriatal administration on Na+,K+- ATPase, CK and AChE activities, on brain metabolism, on glutamate uptake and on reactive oxidative species (ROS) production in striatum of adult rats. Finally, we investigated the effect of intraestriatal administration of GAA on inhibitory avoidance in adult rats. Our results showed that the intrastriatal administration inhibits Na+,K+-ATPase, CK and complex II activities and increases TBARS and AChE activity. We also showed that GAA in vitro inhibits glutamate uptake and induces oxidative stress. GAA administration also impairs memory in adult rats. The inhibition of Na+,K+-ATPase, CK and complex II activities and increase of TBARS production caused by intrastriatal administration of GAA was prevented by vitamins E and C or taurine pretreatment (for one week). Our findings indicate that inhibition of Na+,K+- ATPase, CK and complex II activities and alterations in glutamate uptake and oxidative stress caused by GAA may contribute to the neurological dysfunction characteristic of GAMT-deficiency patients.